Characterization of Anodic Aluminas I: Composition of Films from Acidic Anodizing Electrolytes

High-purity aluminum sheet was anodized in eleven different acidic elec-trolytes: boric, chromic, phosphoric, sulfuric, oxalic, malonic, succinic, tartaric, fumaric, glutaric, and adipic acids. These anodic coatings were analyzed by infrared to determine the composition and nature of each. We found, aside from alumina differences particular to each electrolyte, that the anodic barrier layer can be distinguished from the porous layer by an absorption band in the 1000-900 wavenumber egion. Background information for the interpretation of anodic coating infrared data was obtained from the in-frared analysis of known mineral forms of alumina. A band correlation table was devised. Anodic alumina films are general ly prepared in acidic electrolytes, such as dilute sulfuric acid, and have properties attributable to the specific electrolyte, the anodizing conditions, and the aluminum substrat